2007-08-15 11:28:22 -03:00
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************************************
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Idioms and Anti-Idioms in Python
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************************************
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:Author: Moshe Zadka
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Merged revisions 62350-62355,62358-62359,62364-62365,62370,62372-62375,62378-62379,62381 via svnmerge from
svn+ssh://pythondev@svn.python.org/python/trunk
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r62350 | nick.coghlan | 2008-04-15 12:25:31 +0200 (Tue, 15 Apr 2008) | 1 line
Issue 2439: add pkgutils.get_data() as a convenience wrapper for the PEP 302 get_data() API (contributed by Paul Moore)
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r62351 | nick.coghlan | 2008-04-15 12:28:14 +0200 (Tue, 15 Apr 2008) | 1 line
Add test file missing from rev 62350
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r62352 | benjamin.peterson | 2008-04-15 13:58:46 +0200 (Tue, 15 Apr 2008) | 2 lines
Add myself to Doc/ACKS.txt
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r62353 | andrew.kuchling | 2008-04-15 15:10:07 +0200 (Tue, 15 Apr 2008) | 6 lines
Add *,**,@ to index, as suggested by
http://farmdev.com/thoughts/24/what-does-the-def-star-variable-or-def-asterisk-parameter-syntax-do-in-python-/
The right entry type to use isn't clear; operator seems wrong, because *,**,@
aren't being used in expressions here. I put them as 'statement'; 'syntax'
might be better.
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r62354 | andrew.kuchling | 2008-04-15 15:10:41 +0200 (Tue, 15 Apr 2008) | 1 line
Typo fix
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r62355 | mark.dickinson | 2008-04-15 22:51:18 +0200 (Tue, 15 Apr 2008) | 3 lines
Fix for possible signed overflow: the behaviour of -LONG_MIN is
undefined in ANSI C.
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r62358 | jeroen.ruigrok | 2008-04-16 14:47:01 +0200 (Wed, 16 Apr 2008) | 2 lines
Reformat to 80 columns prior to adding documentation.
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r62359 | jeroen.ruigrok | 2008-04-16 14:57:43 +0200 (Wed, 16 Apr 2008) | 2 lines
Add details about the return value for mmap.flush().
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r62364 | raymond.hettinger | 2008-04-17 12:48:31 +0200 (Thu, 17 Apr 2008) | 1 line
Issue 2648: Add leading zero to money format recipe in the docs.
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r62365 | jeroen.ruigrok | 2008-04-17 14:39:45 +0200 (Thu, 17 Apr 2008) | 2 lines
Be consistent in the use of read-only.
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r62370 | andrew.kuchling | 2008-04-17 22:44:06 +0200 (Thu, 17 Apr 2008) | 1 line
Typo fixes
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r62372 | andrew.kuchling | 2008-04-18 04:40:47 +0200 (Fri, 18 Apr 2008) | 1 line
Use correct parameter name
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r62373 | andrew.kuchling | 2008-04-18 18:53:09 +0200 (Fri, 18 Apr 2008) | 1 line
#2654: fix typo
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r62374 | andrew.kuchling | 2008-04-18 20:28:23 +0200 (Fri, 18 Apr 2008) | 4 lines
Remove personal note from Jim Roskind; it no longer applies, and the
e-mail address is for a previous employer.
Can we move the big long copyright statement into a sidebar or something?
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r62375 | andrew.kuchling | 2008-04-18 20:39:55 +0200 (Fri, 18 Apr 2008) | 1 line
Rewrite introductory section, and remove old section. (It was already commented-out, but why keep it?)
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r62378 | skip.montanaro | 2008-04-18 22:35:46 +0200 (Fri, 18 Apr 2008) | 1 line
resolve issue 2014
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r62379 | benjamin.peterson | 2008-04-18 22:45:33 +0200 (Fri, 18 Apr 2008) | 2 lines
Fix indentation in sysmodule.c
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r62381 | amaury.forgeotdarc | 2008-04-19 01:31:33 +0200 (Sat, 19 Apr 2008) | 3 lines
Some tests did not pass on repeated calls (regrtest -R::)
Perform additional cleanup, mostly deleting from sys.modules, or clearing the warnings registry.
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2008-04-18 21:55:37 -03:00
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This document is placed in the public domain.
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2007-08-15 11:28:22 -03:00
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.. topic:: Abstract
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This document can be considered a companion to the tutorial. It shows how to use
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Python, and even more importantly, how *not* to use Python.
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Language Constructs You Should Not Use
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======================================
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While Python has relatively few gotchas compared to other languages, it still
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has some constructs which are only useful in corner cases, or are plain
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dangerous.
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from module import \*
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---------------------
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Inside Function Definitions
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^^^^^^^^^^^^^^^^^^^^^^^^^^^
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``from module import *`` is *invalid* inside function definitions. While many
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versions of Python do not check for the invalidity, it does not make it more
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valid, no more then having a smart lawyer makes a man innocent. Do not use it
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like that ever. Even in versions where it was accepted, it made the function
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execution slower, because the compiler could not be certain which names are
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local and which are global. In Python 2.1 this construct causes warnings, and
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sometimes even errors.
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At Module Level
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^^^^^^^^^^^^^^^
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While it is valid to use ``from module import *`` at module level it is usually
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a bad idea. For one, this loses an important property Python otherwise has ---
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you can know where each toplevel name is defined by a simple "search" function
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in your favourite editor. You also open yourself to trouble in the future, if
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some module grows additional functions or classes.
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One of the most awful question asked on the newsgroup is why this code::
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f = open("www")
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f.read()
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does not work. Of course, it works just fine (assuming you have a file called
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"www".) But it does not work if somewhere in the module, the statement ``from os
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import *`` is present. The :mod:`os` module has a function called :func:`open`
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which returns an integer. While it is very useful, shadowing builtins is one of
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its least useful properties.
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Remember, you can never know for sure what names a module exports, so either
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take what you need --- ``from module import name1, name2``, or keep them in the
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module and access on a per-need basis --- ``import module; print(module.name)``.
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When It Is Just Fine
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^^^^^^^^^^^^^^^^^^^^
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There are situations in which ``from module import *`` is just fine:
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* The interactive prompt. For example, ``from math import *`` makes Python an
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amazing scientific calculator.
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* When extending a module in C with a module in Python.
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* When the module advertises itself as ``from import *`` safe.
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from module import name1, name2
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-------------------------------
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This is a "don't" which is much weaker then the previous "don't"s but is still
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something you should not do if you don't have good reasons to do that. The
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reason it is usually bad idea is because you suddenly have an object which lives
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Merged revisions 60481,60485,60489-60492,60494-60496,60498-60499,60501-60503,60505-60506,60508-60509,60523-60524,60532,60543,60545,60547-60548,60552,60554,60556-60559,60561-60562,60569,60571-60572,60574,60576-60583,60585-60586,60589,60591,60594-60595,60597-60598,60600-60601,60606-60612,60615,60617,60619-60621,60623-60625,60627-60629,60631,60633,60635,60647,60650,60652,60654,60656,60658-60659,60664-60666,60668-60670,60672,60676,60678,60680-60683,60685-60686,60688,60690,60692-60694,60697-60700,60705-60706,60708,60711,60714,60720,60724-60730,60732,60736,60742,60744,60746,60748,60750-60751,60753,60756-60757,60759-60761,60763-60764,60766,60769-60770,60774-60784,60787-60789,60793,60796,60799-60809,60812-60813,60815-60821,60823-60826,60828-60829,60831-60834,60836,60838-60839,60846-60849,60852-60854,60856-60859,60861-60870,60874-60875,60880-60881,60886,60888-60890,60892,60894-60898,60900-60931,60933-60958 via svnmerge from
svn+ssh://pythondev@svn.python.org/python/trunk
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r60901 | eric.smith | 2008-02-19 14:21:56 +0100 (Tue, 19 Feb 2008) | 1 line
Added PEP 3101.
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r60907 | georg.brandl | 2008-02-20 20:12:36 +0100 (Wed, 20 Feb 2008) | 2 lines
Fixes contributed by Ori Avtalion.
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r60909 | eric.smith | 2008-02-21 00:34:22 +0100 (Thu, 21 Feb 2008) | 1 line
Trim leading zeros from a floating point exponent, per C99. See issue 1600. As far as I know, this only affects Windows. Add float type 'n' to PyOS_ascii_formatd (see PEP 3101 for 'n' description).
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r60910 | eric.smith | 2008-02-21 00:39:28 +0100 (Thu, 21 Feb 2008) | 1 line
Now that PyOS_ascii_formatd supports the 'n' format, simplify the float formatting code to just call it.
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r60918 | andrew.kuchling | 2008-02-21 15:23:38 +0100 (Thu, 21 Feb 2008) | 2 lines
Close manifest file.
This change doesn't make any difference to CPython, but is a necessary fix for Jython.
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r60921 | guido.van.rossum | 2008-02-21 18:46:16 +0100 (Thu, 21 Feb 2008) | 2 lines
Remove news about float repr() -- issue 1580 is still in limbo.
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r60923 | guido.van.rossum | 2008-02-21 19:18:37 +0100 (Thu, 21 Feb 2008) | 5 lines
Removed uses of dict.has_key() from distutils, and uses of
callable() from copy_reg.py, so the interpreter now starts up
without warnings when '-3' is given. More work like this needs to
be done in the rest of the stdlib.
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r60924 | thomas.heller | 2008-02-21 19:28:48 +0100 (Thu, 21 Feb 2008) | 4 lines
configure.ac: Remove the configure check for _Bool, it is already done in the
top-level Python configure script.
configure, fficonfig.h.in: regenerated.
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r60925 | thomas.heller | 2008-02-21 19:52:20 +0100 (Thu, 21 Feb 2008) | 3 lines
Replace 'has_key()' with 'in'.
Replace 'raise Error, stuff' with 'raise Error(stuff)'.
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r60927 | raymond.hettinger | 2008-02-21 20:24:53 +0100 (Thu, 21 Feb 2008) | 1 line
Update more instances of has_key().
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r60928 | guido.van.rossum | 2008-02-21 20:46:35 +0100 (Thu, 21 Feb 2008) | 3 lines
Fix a few typos and layout glitches (more work is needed).
Move 2.5 news to Misc/HISTORY.
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r60936 | georg.brandl | 2008-02-21 21:33:38 +0100 (Thu, 21 Feb 2008) | 2 lines
#2079: typo in userdict docs.
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r60938 | georg.brandl | 2008-02-21 21:38:13 +0100 (Thu, 21 Feb 2008) | 2 lines
Part of #2154: minimal syntax fixes in doc example snippets.
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r60942 | raymond.hettinger | 2008-02-22 04:16:42 +0100 (Fri, 22 Feb 2008) | 1 line
First draft for itertools.product(). Docs and other updates forthcoming.
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r60955 | nick.coghlan | 2008-02-22 11:54:06 +0100 (Fri, 22 Feb 2008) | 1 line
Try to make command line error messages from runpy easier to understand (and suppress traceback cruft from the implicitly invoked runpy machinery)
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r60956 | georg.brandl | 2008-02-22 13:31:45 +0100 (Fri, 22 Feb 2008) | 2 lines
A lot more typo fixes by Ori Avtalion.
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r60957 | georg.brandl | 2008-02-22 13:56:34 +0100 (Fri, 22 Feb 2008) | 2 lines
Don't reference pyshell.
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r60958 | georg.brandl | 2008-02-22 13:57:05 +0100 (Fri, 22 Feb 2008) | 2 lines
Another fix.
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2008-02-22 12:37:40 -04:00
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in two separate namespaces. When the binding in one namespace changes, the
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binding in the other will not, so there will be a discrepancy between them. This
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happens when, for example, one module is reloaded, or changes the definition of
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a function at runtime.
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Bad example::
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# foo.py
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a = 1
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# bar.py
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from foo import a
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if something():
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a = 2 # danger: foo.a != a
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Good example::
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# foo.py
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a = 1
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# bar.py
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import foo
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if something():
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foo.a = 2
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except:
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-------
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Python has the ``except:`` clause, which catches all exceptions. Since *every*
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error in Python raises an exception, this makes many programming errors look
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like runtime problems, and hinders the debugging process.
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The following code shows a great example::
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try:
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foo = opne("file") # misspelled "open"
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except:
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sys.exit("could not open file!")
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The second line triggers a :exc:`NameError` which is caught by the except
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clause. The program will exit, and you will have no idea that this has nothing
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to do with the readability of ``"file"``.
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The example above is better written ::
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try:
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foo = opne("file") # will be changed to "open" as soon as we run it
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except IOError:
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sys.exit("could not open file")
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There are some situations in which the ``except:`` clause is useful: for
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example, in a framework when running callbacks, it is good not to let any
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callback disturb the framework.
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Exceptions
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==========
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Exceptions are a useful feature of Python. You should learn to raise them
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whenever something unexpected occurs, and catch them only where you can do
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something about them.
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The following is a very popular anti-idiom ::
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def get_status(file):
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if not os.path.exists(file):
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print("file not found")
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sys.exit(1)
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return open(file).readline()
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Consider the case the file gets deleted between the time the call to
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:func:`os.path.exists` is made and the time :func:`open` is called. That means
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the last line will throw an :exc:`IOError`. The same would happen if *file*
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exists but has no read permission. Since testing this on a normal machine on
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existing and non-existing files make it seem bugless, that means in testing the
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results will seem fine, and the code will get shipped. Then an unhandled
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:exc:`IOError` escapes to the user, who has to watch the ugly traceback.
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Here is a better way to do it. ::
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def get_status(file):
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try:
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return open(file).readline()
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except (IOError, OSError):
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print("file not found")
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sys.exit(1)
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In this version, \*either\* the file gets opened and the line is read (so it
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works even on flaky NFS or SMB connections), or the message is printed and the
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application aborted.
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Still, :func:`get_status` makes too many assumptions --- that it will only be
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used in a short running script, and not, say, in a long running server. Sure,
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the caller could do something like ::
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try:
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status = get_status(log)
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except SystemExit:
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status = None
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So, try to make as few ``except`` clauses in your code --- those will usually be
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a catch-all in the :func:`main`, or inside calls which should always succeed.
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So, the best version is probably ::
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def get_status(file):
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return open(file).readline()
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The caller can deal with the exception if it wants (for example, if it tries
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several files in a loop), or just let the exception filter upwards to *its*
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caller.
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The last version is not very good either --- due to implementation details, the
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file would not be closed when an exception is raised until the handler finishes,
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and perhaps not at all in non-C implementations (e.g., Jython). ::
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def get_status(file):
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fp = open(file)
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try:
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return fp.readline()
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finally:
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fp.close()
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Using the Batteries
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===================
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Every so often, people seem to be writing stuff in the Python library again,
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usually poorly. While the occasional module has a poor interface, it is usually
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much better to use the rich standard library and data types that come with
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Python then inventing your own.
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A useful module very few people know about is :mod:`os.path`. It always has the
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correct path arithmetic for your operating system, and will usually be much
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better then whatever you come up with yourself.
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Compare::
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# ugh!
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return dir+"/"+file
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# better
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return os.path.join(dir, file)
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More useful functions in :mod:`os.path`: :func:`basename`, :func:`dirname` and
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:func:`splitext`.
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There are also many useful builtin functions people seem not to be aware of for
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some reason: :func:`min` and :func:`max` can find the minimum/maximum of any
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sequence with comparable semantics, for example, yet many people write their own
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:func:`max`/:func:`min`. Another highly useful function is
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:func:`functools.reduce`. A classical use of :func:`reduce` is something like
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::
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2007-09-04 04:15:32 -03:00
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import sys, operator, functools
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nums = map(float, sys.argv[1:])
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print(functools.reduce(operator.add, nums) / len(nums))
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This cute little script prints the average of all numbers given on the command
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line. The :func:`reduce` adds up all the numbers, and the rest is just some
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pre- and postprocessing.
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2007-11-29 13:41:05 -04:00
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On the same note, note that :func:`float` and :func:`int` accept arguments of
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type string, and so are suited to parsing --- assuming you are ready to deal
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with the :exc:`ValueError` they raise.
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2007-08-15 11:28:22 -03:00
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Using Backslash to Continue Statements
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======================================
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Since Python treats a newline as a statement terminator, and since statements
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are often more then is comfortable to put in one line, many people do::
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if foo.bar()['first'][0] == baz.quux(1, 2)[5:9] and \
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calculate_number(10, 20) != forbulate(500, 360):
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pass
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Merged revisions 59605-59624 via svnmerge from
svn+ssh://pythondev@svn.python.org/python/trunk
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r59606 | georg.brandl | 2007-12-29 11:57:00 +0100 (Sat, 29 Dec 2007) | 2 lines
Some cleanup in the docs.
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r59611 | martin.v.loewis | 2007-12-29 19:49:21 +0100 (Sat, 29 Dec 2007) | 2 lines
Bug #1699: Define _BSD_SOURCE only on OpenBSD.
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r59612 | raymond.hettinger | 2007-12-29 23:09:34 +0100 (Sat, 29 Dec 2007) | 1 line
Simpler documentation for itertools.tee(). Should be backported.
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r59613 | raymond.hettinger | 2007-12-29 23:16:24 +0100 (Sat, 29 Dec 2007) | 1 line
Improve docs for itertools.groupby(). The use of xrange(0) to create a unique object is less obvious than object().
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r59620 | christian.heimes | 2007-12-31 15:47:07 +0100 (Mon, 31 Dec 2007) | 3 lines
Added wininst-9.0.exe executable for VS 2008
Integrated bdist_wininst into PCBuild9 directory
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r59621 | christian.heimes | 2007-12-31 15:51:18 +0100 (Mon, 31 Dec 2007) | 1 line
Moved PCbuild directory to PC/VS7.1
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r59622 | christian.heimes | 2007-12-31 15:59:26 +0100 (Mon, 31 Dec 2007) | 1 line
Fix paths for build bot
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r59623 | christian.heimes | 2007-12-31 16:02:41 +0100 (Mon, 31 Dec 2007) | 1 line
Fix paths for build bot, part 2
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r59624 | christian.heimes | 2007-12-31 16:18:55 +0100 (Mon, 31 Dec 2007) | 1 line
Renamed PCBuild9 directory to PCBuild
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2007-12-31 12:14:33 -04:00
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You should realize that this is dangerous: a stray space after the ``\`` would
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2007-08-15 11:28:22 -03:00
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make this line wrong, and stray spaces are notoriously hard to see in editors.
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In this case, at least it would be a syntax error, but if the code was::
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value = foo.bar()['first'][0]*baz.quux(1, 2)[5:9] \
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+ calculate_number(10, 20)*forbulate(500, 360)
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then it would just be subtly wrong.
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It is usually much better to use the implicit continuation inside parenthesis:
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This version is bulletproof::
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value = (foo.bar()['first'][0]*baz.quux(1, 2)[5:9]
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+ calculate_number(10, 20)*forbulate(500, 360))
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